Detectives have been granted more time to question four men arrested under the Terrorism Act, police said.
The men, three aged 19 and one 24, are being held on suspicion of fundraising in the UK for terrorist purposes and of travelling to Pakistan for terrorist training.
They were arrested at their homes in the Sparkhill area of Birmingham by officers from the West Midlands Counter Terrorism Unit on Tuesday.
West Midlands Police said the arrests were pre-planned and were made by unarmed officers as part of a major ongoing investigation.
A spokesman said the arrests were "not made in response to any immediate threat to public safety."
Officers now have until next Tuesday morning to charge or release the men, or apply for another warrant of further detention.
Police said the men were arrested as part of Operation Pitsford, an ongoing and large-scale inquiry.
Eight people arrested in September have already been charged in connection with the investigation and seven of them are currently on remand after appearing in court.